In the field of circuit-forming techniques for electric devices or electronic devices, various conductive pastes are used when mounting electronic components onto a substrate. The conductive paste is a composition containing a conductive filler component and a resin component, and an electrical insulating resin is generally used for the resin component. Although the resin component does not exhibit electric conductivity, by curing and shrinking the resin component, the conductive fillers are brought into contact with or into proximity to each other, rendering the composition as a whole conductive.
By increasing the conductive filler content in the conductive paste, and increasing the contact area or contact probability between the conductive filler particles, the resistance of the conductive paste can be reduced. However, it is not sufficient to improve the contact state only in order to reduce the resistance of the conductive paste. Also, when electronic components are bonded to a substrate using a conductive paste and a thermal cycle reliability test is carried out, the resistance value of the joint portion after such a test increases compared with the initial value.
Thus, the use of a combination of a high melting point conductive filler and a low melting point conductive filler has been proposed. The low melting point conductive filler is melted at its melting point or higher temperatures and attached to the high melting point conductive filler, and forms a chain structure of the coupled filler particles. As a result, the mechanical strength of the joint portion is increased, and the reliability of the electrical connection is also increased to a certain degree (ref. Patent Document 1: Japanese Laid-Open Patent Publication No. Hei 10-279903).
It is also proposed that the surface of the conductive filler is melted and the conductive filler particles are welded to each other at the time of heating to cure the conductive paste. By welding the conductive filler particles, the reliability of the electrical connection at the joint portion is increased to a certain degree (ref. Patent Document 2: Japanese Laid-Open Patent Publication No. 2005-89559).
On the other hand, use of a conductive paste containing a thermosetting resin and a conductive filler as a potting compound for the via holes of a circuit board has been proposed. The conductive filler consists of low melting point metal particles with a melting point of 230° C. or less. The low melting point metal particles take the role of an electrical connection similar to solder, and the thermosetting resin reinforces the joint portion (ref. Patent Document 3: Japanese Laid-Open Patent Publication No. 2005-71825).
When electronic components are bonded to a substrate using a conductive paste, solder balls may be generated at the time of soldering. According to the results of various examinations conducted by the inventors, when the conductive paste contains two or more types of metal particles, the amount of the solder balls generated tends to be large, and this tendency is affected by fluxes. Particularly, a conductive paste containing a thermosetting resin and metal particles as conductive filler brings about dipping of the thermosetting resin easily, unlike a general soldering paste. Therefore, solder balls easily flow out from between the substrate and component electrodes. Furthermore, because the conductive paste containing a low melting point flux component has poor storage stability, and its viscosity varies greatly at ambient temperature, the handling of the paste is troublesome. Thus, there is an increasing demand for a conductive paste that does not easily generate solder balls when soldering, has excellent storage stability, and is capable of securing an excellent electrical connection.